The invention relates to a spool for strip-form recording substrates, in particular for photographic films, with a slit arranged in the spool core and means provided therein for fixing the end of the strip which is pushed into the slit, wherein transitional surfaces which are rounded approximately in the manner of a cylinder surface are provided at the inlet end between the axially running upper and lower slit faces and the external surfaces of the spool core.
A spool of this type is known from DE-AS 10 58 363. In the spool disclosed therein, however, the transitional surfaces adjoining the slit walls on the inlet side are curved very markedly and extend in cross-section only over an angular range of about 90.degree. and kinks have to be accepted between these transitional surfaces and the external surfaces of the spool core.
This is revealed more clearly in the previously known embodiment according to US-PS 24 77 010, in which the angular range of the transitional surfaces is even smaller.
It has been found that films which have been wound onto film spools of this type frequently cannot withstand relatively high tensile stresses when being unwound again. On the other hand, however, films which, after image-wise exposure, are introduced by the user for unwinding and copying into processing devices designed for this purpose, are mechanically unwound after mechanically removing the roll of film including spool core from the film cassette in so-called presplicer devices and are stuck together to form long strips for the purpose of continuous processing, the beginning of the film being stuck to the end of a respective preceding film. To achieve high running speeds and thus for the economic use of the machinery involved in processing, the film is also unwound from the supply spool at high speed. As the end of the film is firmly hooked to the spool core, the film suddenly comes to a stand-still when being unwound from the spool at the end of the unwinding process which takes place at high speed. However, the end of the film should still not tear from the spool as it is to be automatically gripped in the presplicer device for the subsequent adhesion process and, after mechanical separation of the unexposed end portion of the film adhering to the spool, is to be supplied to the adhesion unit where it is connected to the beginning of the next film. The sudden stoppage of film travel at the end of the unwinding process just described leads to quite considerable dynamic tensile stresses to which the film is subjected with its end portion fixed on the spool. Moreover, in the previously known embodiments, the film is also strongly stressed statically at the end of the unwinding process in the camera and in the presplicer device because here the film does not come to rest on the inlet surfaces of the spool slit or at least comes to rest on an edge of the film slit. Although the film is supported here, the support is restricted to edge contact.